Beyond (Ga,Mn)As: µSR reveals microscopic magnetic behaviors of next-generation bulk diluted ferromagnetic semiconductor Li(Zn,Mn)As

Background: Traditional III-V diluted ferromagnetic semiconductor (Ga,Mn)As remains the benchmark platform for semiconductor spintronics, e.g., a new generation of low-power, high-efficiency electronic computer integrating storage and computing, yet it suffers critical drawbacks: mandatory thin-film synthesis via molecular beam epitaxy, air instability, coupled spin-charge doping that complicates mechanism disentanglement, and unfeasible bulk neutron scattering characterization. Proposed in 2007 and successfully synthesized as bulk crystals in 2011, Li(Zn,Mn)As is an isostructural cubic F-43m I-II-V DMS featuring fully decoupled spin doping (Mn²⁺ substituting Zn²⁺) and charge doping (excess Li carriers). Unlike (Ga,Mn)As, bulk polycrystalline Li(Zn,Mn)As is chemically stable in ambient air, making it ideal for local-probe microscopic magnetic characterization. Prior work confirmed ferromagnetic ordering emerges only under simultaneous carrier and Mn spin co-doping, yet the microscopic evolution from paramagnetism to long-range ferromagnetism, intrinsic magnetic ground state nature, and critical spin dynamics near Curie temperature (T_C) remained unquantified until this µSR investigation.

The Solution: The research team performed zero-field (ZF), weak transverse-field (wTF) and longitudinal-field (LF) µSR experiments at TRIUMF National Laboratory (Canada) on four compositional variants of Li_1+y(Zn_1-xMn_x)As spanning undoped paramagnetic reference to high-Mn ferromagnetic samples. A refined global fitting framework integrating Lorentzian Kubo-Toyabe stretched exponential models was developed to separate static ordered magnetic volume fraction (f_M) and dynamic spin fluctuation contributions across full temperature and magnetic field windows, overcoming limitations of truncated time-window analysis adopted in earlier DMS µSR studies. Three definitive, quantitative experimental advances emerge from the work:

1. The first µSR-derived percolation threshold for homogeneous long-range ferromagnetic order is determined at ~5% Mn substitution under fixed 10% excess Li carrier doping. At low Mn loading (3%), only 56% of the sample volume forms static ferromagnetic domains with coexisting dynamic paramagnetic regions; 5% Mn doping yields 100% fully ordered static magnetic volume at base temperature (1.8 K), confirming complete magnetic homogeneity above this critical spin concentration threshold.
2. LF-µSR decoupling measurements unambiguously distinguish two distinct magnetic ground states. Fully homogeneous samples (x=0.05 Mn) exhibit purely static local magnetic moments at low temperature, while phase-separated low-Mn specimens host concurrent static ordered domains and fluctuating dynamic spin environments. This dual ground-state behavior directly explains previously observed deviations from universal a_s-T_C scaling curves shared by cubic DMS families including (Ga,Mn)As, Li(Zn,Mn)P and Na(Zn,Mn)Sb. etc.
3. Longitudinal-field relaxation rate (1/T₁) data rule out conventional critical spin slowing down near the transition temperature for both homogeneous and phase-separated ferromagnetic compositions. Unlike canonical spin-glass DMS systems that display sharp 1/T₁ critical peaks at transition temperature, Li(Zn,Mn)As shows only mild relaxation anomalies at T_C, indicating suppressed critical spin dynamics independent of magnetic homogeneity. This unexpected behavior partially aligns with bound magnetic polaron percolation frameworks but introduces unresolved contradictions with standard 3D disordered Heisenberg magnet transition models.

“For over the past 15 years, the field lacked direct microscopic quantification of how magnetic order nucleates and expands within Li(Zn,Mn)As, the flagship bulk DMS that decouples spin and charge doping,” pointed by the team. “Muons act as atomic-scale local magnetic sensors sensitive to both static internal field distributions and fast spin fluctuations; our multi-field µSR design allowed us to disentangle static vs dynamic contributions that ZF/wTF spectroscopy alone cannot isolate. The 5% Mn percolation threshold we measured provides a concrete experimental benchmark to validate BMP overlap percolation theories, while the absence of critical spin fluctuations near T_C forces us to revisit standard assumptions about second-order magnetic phase transitions in heavily disordered dilute ferromagnets. Our scaling analysis linking static µSR relaxation rate a_s and T_C unifies magnetic interaction mechanisms across all cubic F-43m DMS and related systems, establishing a universal metric to evaluate magnetic homogeneity in future spintronic candidate materials.”

The Future: The team clarifies that bound magnetic polaron theory offers partial qualitative interpretation for the observed doping-dependent magnetic evolution, yet cannot fully account for the suppressed critical dynamics at the paramagnet-ferromagnet transition. Follow-up research directions outlined in the paper include high-resolution transport, heat capacity and magnetization hysteresis measurements to resolve the true order of the magnetic phase transition, fine-grained Mn doping series to refine the percolation threshold value, and extension of LF-µSR characterization to high- T_C BaZn₂As₂-family bulk DMS single crystals. Long-term translational goals include designing Li(Zn,Mn)As-based heterostructures that exploit controllable magnetic phase separation for low-power spintronic device prototypes, alongside in-situ µSR experiments under electric field or pressure stimuli to dynamically tune ferromagnetic ordering.

The Impact: This work represents the first systematic microscopic investigation utilizing µSR to elucidate the magnetic ground states, phase evolution, and critical spin dynamics of both homogeneous and inhomogeneous ferromagnetic phases within the next generation DMS Li(Zn,Mn)As system, alongside a direct side-by-side comparison of these two regimes under a unified physical framework. This study further validates both the applicable scope and inherent limitations of the phenomenological bound magnetic polaron (BMP) model. Methodologically, this research establishes the first full-spectrum quantitative fitting protocol for wTF-µSR measurements on DMSs, delivering a standardized analytical benchmark for follow-up related investigations.

The research has been recently published in the online edition of Materials Futures, a prominent international journal in the field of interdisciplinary materials science research.

Citation: Guoqiang Zhao, Zheng Deng, Baosen Min, Timothy Ziman, Gang Su, Bo Gu, Changqing Jin, Yasutomo J Uemura. Magnetic ground state, evolution, and critical dynamics in the new generation diluted ferromagnetic semiconductors Li(Zn,Mn)As: a μSR perspective[J]. Materials Futures, 2026, 5(4): 045201. DOI: 10.1088/2752-5724/ae84ef